Preheat the oven:

• Preheat your oven to 350°F (175°C). Grease and flour an 8-inch square or round baking pan.

Prepare the custard layer:

• In a saucepan, whisk together the milk, sugar, egg yolks, vanilla extract, melted butter, and cornstarch.
• Cook the mixture over medium heat, stirring constantly, until it thickens and starts to bubble.
• Once thickened, remove from the heat and set aside to cool slightly.

Prepare the cake layer:

• In a large mixing bowl, beat the egg whites with a pinch of salt until soft peaks form.
• In a separate bowl, beat the egg yolks and sugar together until pale and creamy. Add the milk and vanilla extract and mix until combined.
• Sift in the flour and baking powder, mixing until the batter is smooth.
• Gently fold in the beaten egg whites into the egg yolk mixture, being careful not to deflate the meringue.

Assemble the cake:

• Pour the cake batter into the prepared pan, smoothing it into an even layer.
• Carefully pour the custard mixture over the top of the cake batter. The custard will settle at the bottom while baking.

Bake the cake:

• Bake for 40-45 minutes or until the top is lightly golden and a toothpick inserted into the center comes out clean.
• Once baked, remove from the oven and let it cool in the pan for 15 minutes. Then, transfer to a wire rack to cool completely.

Serve and enjoy:

• Once cooled, slice the cake and serve. The cake will have a fluffy texture with a creamy custard layer underneath.